The present invention relates to semiconductor devices and their methods of fabrication. More particularly, the present invention relates to the processing of copper interconnect material and the resultant device utilizing the same. Even more particularly, the present invention relates to reducing electromigration in copper interconnect lines by doping their surfaces with barrier material using wet chemical methods.
Currently, the semiconductor industry is demanding faster and denser devices (e.g., 0.05-xcexcm to 0.25-xcexcm) which implies an ongoing need for low resistance metallization. Such need has sparked research into resistance reduction through the use of barrier metals, stacks, and refract or metals. Despite aluminum""s (Al) adequate resistance, other Al properties render it less desirable as a candidate for these higher density devices, especially with respect to its deposition into plug regions having a high aspect ratio cross-sectional area. Thus, research into the use of copper as an interconnect material has been revisited, copper being advantageous as a superior electrical conductor, providing better wettability, providing adequate electromigration resistance, and permitting lower depositional temperatures. The copper (Cu) interconnect material may be deposited by chemical vapor deposition (CVD), plasma enhanced chemical vapor deposition (PECVD), sputtering, electroless plating, and electrolytic plating.
However, some disadvantages of using Cu as an interconnect material include etching problems, corrosion, and diffusion into silicon.1 These problems have sparked further research into the formulation of barrier materials for preventing electromigration in both Al and Cu interconnect lines. In response to electromigration concerns relating to the fabrication of semiconductor devices having aluminum-copper alloy interconnect lines, the industry has been investigating the use of various barrier materials. For instance, titanium-tungsten (Tixe2x80x94W) and titanium nitride (TiN) layers have been used as well as refractory metals such as titanum (Ti), tungsten (W), tantalum (Ta), and molybdenum (Mo) and their silicates.2 Although the foregoing materials are adequate for Al interconnects and Alxe2x80x94Cu alloy interconnects, they have not been entirely effective with respect to all-Cu interconnects. Further, although CVD has been conventionally used for depositing other metal(s) on an interconnect surface, CVD is not a cost-effective method of doping Cu interconnect surfaces with Ca ions. Therefore, a need exists for providing a method of fabricating a semiconductor device having Cu interconnect lines whose surfaces are doped with Ca ions for preventing electromigration and a device thereby formed.
1Peter Van Zant, Microchip Fabrication: A Practical Guide to Semiconductor Processing, 3rd Ed., p. 397 (1997). 2Id., at 392. 
Accordingly, the present invention provides a method of fabricating a semiconductor device having Cu interconnect lines, formed in vias of a semiconductor substrate, whose surfaces are selectively doped with calcium (Ca) ions for preventing electromigration and a device thereby formed. The present invention method reduces electromigration in Cu interconnect lines by restricting Cu diffusion pathways along the interconnect surface. This diffusion restriction is achieved by selectively doping the Cu interconnect surfaces with Ca ions from a chemical solution. More specifically, the present invention provides a method of fabricating a semiconductor device having a contaminant-reduced Cuxe2x80x94Ca/Cu interconnect line structure for reducing electromigration, improving interconnect reliability, and preventing corrosion, the method comprising: (a) providing a semiconductor substrate; (b) depositing a Cu interconnect line on the semiconductor substrate; (c) treating the Cu interconnect line in a chemical solution for facilitating selective doping of the Cu interconnect line with copper and calcium, thereby selectively forming a Cuxe2x80x94Caxe2x80x94X film on the Cu interconnect line, wherein X denotes at least one contaminant; (d) processing the Cuxe2x80x94Caxe2x80x94X film by sputtering under an argon (Ar) atmosphere, thereby effecting a thin Cuxe2x80x94Ca film on the Cu interconnect line; (e) annealing the thin Cuxe2x80x94Ca film, whereby the thin Cuxe2x80x94Ca film is alloyed, thereby forming a contaminant-reduced Cuxe2x80x94Ca alloy surface on the Cu interconnect line, and thereby forming a contaminant-reduced Cuxe2x80x94Ca/Cu interconnect line structure, comprising the contaminant-reduced Cuxe2x80x94Ca alloy surface, on the semiconductor substrate; and (f) completing formation of the semiconductor device. The annealing step primarily removes O and secondarily removes C and S, especially when performed in an environment such as a vacuum, an inert gas, or a reducing ambient such as an ammonia (NH3) plasma. Thus, the resultant device comprises a distinctive contaminant-reduced Cu surfaces selectively doped with calcium (Ca) on Cu interconnect lines formed in the vias of the substrate.